The club legend, who will retire at the end of the campaign, climbed off the bench in the latter stages before producing an incredible goal-of-the-season contender on the opening day with an age-defying bicycle kick which left Marc-Andre ter Stegen helpless.
Barcelona had hit the woodwork twice in the first half but were below-par in general and also lost Luis Suarez to injury as their latest title defence got off to a miserable start.
Athletic, meanwhile, have beaten Barcelona at home for the first time since 2013 as their own campaign began in dream fashion.
There were a number of notable absentees from the Barcelona starting XI, with Lionel Messi injured, Sergio Busquets, Ivan Rakitic and Samuel Umtiti all on the bench and Philippe Coutinho watching on from the stands after news broke moments before kickoff that the club had agreed terms over a loan deal with Bayern Munich.
There were debuts for big-money summer signings Antoine Griezmann and Frenkie de Jong, but it was Athletic who made the brighter start to the match, with Inaki Williams in particular looking dangerous.
The winger - linked with Manchester United before signing a new nine-year contract earlier this week - collected a loose pass from Ousmane Dembele seven minutes in and immediately let fly from range, forcing Ter Stegen into a smart diving save.
Williams came close again when he broke into the Barcelona box on the quarter-hour mark, but once again Ter Stegen was equal to his effort as he got down well to save.
Unai Lopez was the next to call Ter Stegen into action from range, although the Spanish midfielder almost gifted Barcelona the lead against the run of play moments later when his back-pass went straight to Suarez, who swivelled on the ball and sent his effort crashing against the post.
The despair of missing a chance he would have expected to score was exacerbated by Suarez also picking up an injury in the process, forcing the Uruguayan off the field to be replaced by Rafinha as he joined strike partner Messi on the injured list.
Rafinha hit the woodwork himself shortly before the interval as Barcelona once again came within inches of what would have been an undeserved lead, curling a stunning strike towards goal which Unai Simon did brilliantly to tip against the crossbar.
Ernesto Valverde was unhappy with his side's first-half performance and called upon Rakitic to replace Carles Alena at the break, and the Croatian midfielder threatened within five minutes of his introduction as his looping header from a corner landed on the roof of the net.
The change allowed Barca to gain a greater degree of control over the opening stages of the second half, and Rafinha saw another strike pushed wide before Griezmann nodded the resulting corner narrowly off target.
Athletic were more than holding their own against the champions, though, and Lopez fired another long-range effort over before Williams got in behind and shot wide.
The hosts began to defend deeper as the game wore on in an attempt to hold on to the goalless draw, but that tactic almost backfired when the ball broke to Rakitic inside the area, only for the midfielder to blaze his finish over.
Rafinha then saw a shot deflected behind for a corner after some quick footwork inside the box, and Griezmann also pushed for a late winner with a couple of efforts which missed the target.
The glory would go to Athletic stalwart Aduriz, though, as he made a fairytale start to his final season for the club by scoring a sensational winner less than a minute after coming off the bench.
There was some consternation amongst the fans when Williams was taken off for the veteran, but that was quickly forgotten as he rose to meet a cross from the right with a perfectly-timed bicycle kick which flew past a helpless Ter Stegen.
Barca did almost salvage a dramatic point in stoppage time when Clement Lenglet met a free kick and turned his header goalwards, but Simon was on hand to tip it over and ensure that all three points went to the hosts.
ATHLETIC BILBAO (4-2-3-1): Simon; Zapa, Nunez, Yeray, Berchiche; Lopez (Benat 81'), Garcia; De Marcos (Sancet 66'), Garcia, Muniain; Williams (Aduriz 88')
BARCELONA (4-3-3): Ter Stegen; Semedo, Pique, Lenglet, Alba; Roberto (Perez 76'), De Jong, Alena (Rakitic 46'); Dembele, Suarez (Rafinha 37'), Griezmann